1. Field of the Invention
The present invention relates to output circuits for extracting electromagnetic energy from a bunched electron beam, and more particularly, to a novel extended interaction output circuit of a relativistic klystron where the electromagnetic energy is extracted from a linear beam over a broad band of frequency.
2. Description of Related Art
Linear beam tubes are used in sophisticated communication and radar systems which require amplification of an RF or microwave electromagnetic signal. A conventional klystron is an example of a linear beam microwave amplifier. A klystron comprises a number of cavities divided into essentially three sections: an input section, a buncher section and an output section. An electron beam is sent through the klystron, and is velocity modulated by an RF electromagnetic input signal that is provided to the input section. In the buncher section, those electrons that have had their velocity increased gradually overtake the slower electrons, resulting in electron bunching. The traveling electron bunches represent an RF current in the electron beam. The RF current induces electromagnetic energy into the output section of the klystron as the bunched beam passes through the output cavity, and the electromagnetic energy is extracted from the klystron at the output section.
The development of high power klystron amplifiers which operate at a peak power level higher in relation to its pulse length and frequency than that of conventional klystrons has resulted in beam voltage levels generally higher than that previously achieved. To avoid RF breakdown in the output section due to the high beam voltage, multi-cavity output circuits were developed. The multi-cavity output circuits, known as extended interaction output circuits (EIOC), have the advantage that the electromagnetic energy can be removed from the electron beam at a reduced voltage across several gaps over a bandwidth which is greater by an amount that varies inversely with the output circuit impedance level. An example of a high performance extended interaction output circuit is disclosed in U.S. Pat. No. 4,931,695, which is incorporated herein by reference.
In order to achieve an efficient energy exchange between the electron beam and the output circuit, the electromagnetic wave that travels within the output circuit must synchronize with the beam with respect to the velocity of propagation. The '695 patent discloses the use of a multi-cavity extended interaction output circuit utilizing coupling irises to couple adjacent cavities. The dimensions and the locations of the irises can be selected to reduce the effective velocity of propagation of the electromagnetic wave in such a way that the phase velocity of the electromagnetic wave matches with that of the velocity modulated electron beam as it travels from one cavity gap center to the next cavity gap center.
However, conventional multi-cavity output circuits are inefficient when used with high power klystron amplifiers having relativistic electron beams. A relativistic electron beam travels much closer to the velocity of light than conventional klystron electron beams. In a conventional klystron, as in the '695 patent, the velocity of the electromagnetic wave is much slower than the velocity of light, and as the circuit is adjusted to increase the phase velocity, the bandwidth decreases.
Synchronization between the phase velocity of an electromagnetic wave and an accelerated beam at relativistic velocities has been previously demonstrated in association with disk-loaded waveguides. Disk-loaded waveguides are described in Chu and Hansen, The Theory of Disk-Loaded Waveguides, Journal of Applied Physics, volume 18, page 996 (1947). A disk-loaded waveguide has a sequence of cylindrical cavity resonators separated by disks having coupling holes. The disks are equidistant and the coupling hole diameters are the same for all disks, resulting in identical sequential cavities. The coupling holes permit the transmission of an accelerated beam through the waveguide. An equivalent filter network circuit for a fundamental disk-loaded waveguide is disclosed in Chodorow and Nalos, The Design of High-Power Traveling-Wave Tubes, Proceedings of the IRE 649 (May 1956).
In a disk-loaded waveguide, the introduction and selective placement of the disks permits the reduction of the phase velocity of the electromagnetic wave by as much as desired. As the holes in the disks are increased in size, the phase velocity first approaches and then exceeds that of light, and these characteristics are maintained over a fairly large bandwidth. Thus, disk-loaded waveguides are particularly applicable to the acceleration of electrons or protons in a linear accelerator.
Accordingly, it would be desirable to provide an output circuit for use with a relativistic klystron that provides the broad bandwidth characteristics of a multi-cavity extended interaction output circuit and the phase velocity synchronization characteristics of a disk-loaded waveguide. It would be further desirable to provide an output circuit having the above characteristics, while being relatively simple to design and cost effective to fabricate.